1. Field of Invention
The invention relates to a method and apparatus for tracking targets from which radar signals are reflected in a direct path and in a multipath to a receiver.
2. Description of the Related Prior Art
Radar has long been used to determine the elevation and distance of objects of interest such as airplanes or missiles. A radar signal is directed toward the object and is reflected back to a receiver. If the reflected radio waves travel in a direct path from the target to the receiver, the determination of the distance and elevation of the target is relatively easy. However, if some of the reflected radar waves travel in a direct path to the receiver and other reflected radar waves are again reflected off a second surface and thus travel a multipath to the receiver serious errors can occur. Multipath problems frequently occur when surface based radar is used to track a low flying object. When such objects are at low angles and especially when the target is within a beamwidth or so of the horizon, the errors are especially acute.
A number of different techniques for overcoming or reducing multipath elevation errors have been proposed or investigated. Some of the solutions involved special antenna configurations which permit a single pulse solution. Others have proposed for amplitude monopulse signal processing of the usually neglected component of the difference signal that is in phase quadrature with the sum signal in addition to the in-phase component that is normally used. For a phase monopulse signal they propose to use the usually neglected amplitude component of the ratio of half-array signals in addition to the phase component that is normally used.
Most of the proposed solutions to the problem are based upon a simple model of specular reflection, which assumes an image target at a well defined position below the actual target. Furthermore, the art has concluded that extremely accurate elevation data (one hundredth beamwidth RMS) cannot be expected on targets within two beamwidths of the surface, although that accuracy is available for azimuth measurements if signal fading can be avoided.
Thus, there is a need for a method and apparatus for low angle radar tracking which overcomes the problem of tracking targets which reflect radar signals which travel in both a direct path and a multipath.